Zinc Phosphate vs. Iron Phosphate vs. Zinc Plating

Question:

I am a product engineer for an irrigation company, and I am trying to specify a pretreatment process for powder coating. This particular application will demand above average corrosion resistance. This means that the product would be exposed directly to the environment. Basically my decision boils down to selecting between a zinc phosphate and powder coating versus zinc plating and powder coating. We are currently using zinc phosphate and are looking into changing vendors, but our new potential vendor is not set up to run zinc phosphate, only iron phosphate. But he is offering to run zinc plating in its place.

What do you think withstands corrosion more effectively and why?

a) Zinc phosphate + powder coating
b) Iron phosphate + powder coating
c) Zinc plating + powder coating

Also, do you know where I can find information on this topic, specifically where can I find the hours that each process withstands in the salt spray test? L. G.

Answer:

It is pretty safe to assume that your substrate is ferrous, most probably steel. First let’s begin with the basics. All these methods of pretreatment will work on ferrous products. All provide corrosion resistance, although to different degrees. However, there are distinct differences. We will examine each pretreatment individually:

Iron Phosphate: This pretreatment is the most popular pretreatment used on ferrous products. It provides both corrosion resistance and better adhesion for the powder coating than the raw substrate. It is considered non-hazardous, and can be dumped to drain in most parts of the country after the oils are removed. Depending upon the number of stages in the pretreatment system and if a sealer is used, this pretreatment can provide between 250 hours and 500 hours of salt spray corrosion protection over steel. It also provides excellent adhesion for the powder coating.

Zinc Phosphate: This pretreatment is also used on both raw ferrous and galvanized substrates. It is most effective as a pretreatment over galvanized steel. It provides both corrosion resistance and better adhesion for the powder coating than the raw substrate. This material is considered hazardous and must be processed through a wastewater treatment system before dumping to drain. Depending upon the coating weight of the zinc phosphate, this pretreatment can provide 750 hours of corrosion protection over raw steel and up to 1,000 hours of corrosion protection over galvanized steel. It also provides excellent adhesion for the powder coating.

Zinc Plating: This material is not normally used as a pretreatment for powder coating. As an inorganic coating, it has excellent corrosion resistance and mechanical properties when used on ferrous substrates. The process used to apply this product to the substrate generates hazardous waste, as zinc phosphate, so a wastewater system will need to be employed. This material provides no adhesion properties to the substrate for powder coating. It will probably degrade the adhesion of the substrate. This material will provide the best corrosion resistance of the three, but it will sacrifice the adhesion of the powder coating to the substrate. You would be better off not powder coating over this material unless you rough-up the surface beforehand.